This invention relates to Heat Engines in general and more specifically to engines utilizing the shape memory properties of alloys.
Thermally-related shape memory properties of certain nickel-titanium alloys are well known. Such properties were first observed in the 1960's and since their discovery, they have been applied to thermal-mechanical energy conversion systems, such as the one presented in U.S. Pat. No. 3,403,238 by W. J. Buehler et. al. The name nitinol has been generally applied to such alloys. Nitinol is derived from the combination of the first two letters of nickel "ni", the first two letters of titanium "ti", and the letters "nol" from the U.S. Naval Ordiance Laboratory where its properties were first discovered.
Although the shape memory characteristics of nitinol are well known, a brief description is in order. Nitinol, when annealed into a particular shape, will tend to return to that shape when heated above a certain transition temperature. In returning to that shape the nitinol can release forces as great as 55 tons per square inch, and the force of the change occurs uniformly throughout the nitinol element. If cooled below a second transition temperature, the nitinol becomes malleable, that is, bendable with very little resistance.
Although, the reasons why nitinol exhibits such properties are still being investigated, the most common thought is that when the material is annealed, it forms a cubic atomic structure, and when the material is cooled below a certain transition temperature, a martensitic transformation takes place and the nitinol shifts from an initial cubic structure to a complex rhombic structure, making it malleable. When heated above a second entirely different transition temperature, the reverse transformation takes place, from rhombic to cubic, and the material will return to its original shape.
The temperature differential between the upper transformation point when the material returns to its original shape and the lower transformation temperature when the material becomes extremely malleable, is commonly referred to as the temperature gradient. If the nitinol is composed of 55% nickel and 45% titanium, the material will give its shape memory response at a temperature gradient within room temperature range. Slight increases in the proportion of titanium cause the temperature gradient to shift upward such that the material will perform its shape memory response at temperatures as high as 120.degree. centigrade. Studies have revealed that temperature gradients of as little as 9.degree. centigrade can cause the shape memory transformation. Other studies have shown that the transformation can be repeated almost ad infintum with little or no deterioation in nitinol elements after tens of millions of cycles.
The use of nitinol in heat engines is most advantageous because low temperature thermal energy across a small temperature gradient can be used to perform mechanical work. The sources of low grade heat are wide-spread, such as: geothermal energy, oceantic thermal gradient energy, and most notably, solar thermal energy. The invention presented herein has the advantage in that it can utilize any convenient source of thermal energy for its operation. In other words, the thermal energy delivered to the nitinol elements of this invention can be delivered by sunlight or heated fluids such as air.
Other advantages and attributes of this invention will be apparent upon a review of the drawings and a reading of the text hereinafter.